The sulfur present in hydrocarbon crudes including natural gas exists as hydrogen sulfide or, in the initial processing steps, is converted in the main to hydrogen sulfide. The hydrogen sulfide, as part of the gas stream, is normally passed through an absorption system, such as an alkanolamine or a physical absorbent, which concentrates it and separates it from other components of the gas stream. The concentrated hydrogen sulfide gas stream is fed to a Claus plant wherein a portion of the hydrogen sulfide is combusted in the presence of oxygen introduced as air, to form sulfur dioxide in proportions for the reaction: EQU 2H.sub.2 S+SO.sub.2 .fwdarw.3S+2H.sub.2 O
As fast as sulfur dioxide is formed, it begins to react with hydrogen sulfide in the thermal reaction zone to form sulfur. Sulfur formed is condensed from the gas stream in a waste heat boiler and the balance of the gas stream, at the proper stoichiometric ratio of hydrogen sulfide to sulfur dioxide, is passed to one or more catalytic conversion zone(s), typically three, where additional sulfur is formed by the same reaction. As thermodynamics favor reaction at reduced temperatures, only a limited amount of conversion is achieved in each catalytic bed. The formed sulfur is recovered by condensation and the gas reheated for introduction to a following catalytic bed. The catalysts typically used are alumina or bauxite. Conversion efficiencies of from 95% to 97% can be achieved in the Claus plant and, if pollution requirements so dictate, a clean-up operation such as that described in U.S. Pat. No. 3,752,877 to Beavon, incorporated herein by reference, may be used to increase overall conversion to 99.9%-plus. This operation is also applicable in ammonia burning Claus sulfur plant as described in U.S. Pat. No. 4,038,036 to Beavon incorporated herein by reference.
The oxygen required to convert the hydrogen sulfide to sulfur dioxide is usually supplied with air. This results in the introduction of approximately 79 volumes of nitrogen for every 21 volumes of oxygen needed for oxidation of the hydrogen sulfide. The nitrogen does not benefit the process and actually results in having to use larger and more expensive equipment in the Claus sulfur plant. The amount of nitrogen passing through the plant may be reduced by employing pure oxygen or oxygen enriched air. However, this results in higher temperatures in the Claus reaction furnace and necessitates more expensive refractories and other materials of construction in new plants, whereas for existing plants it requires expensive changes in the design and operation of the plant.
The composition of the hydrogen sulfide gas stream will vary from plant to plant and may vary considerably during the life of the plant. In addition to the H.sub.2 S, the Claus plant feed gas may also contain varying amounts of ammonia, hydrocarbons, and other compounds which will react with oxygen and, therefore, affect the temperature increase in the Claus plant reaction furnace when making the change from air to oxygen enriched air up to pure oxygen.
While particularly applicable to new plants, the present invention is also directed to existing Claus sulfur plants in order to increase their capacity by the use of oxygen or oxygen enriched air.